Non-aqueous electrolyte secondary batteries including lithium ion secondary batteries having a high energy density are widely used as power sources for portable appliances such as personal computers, cell phones, digital cameras and camcorders. From environmental and resource protection perspectives, electric vehicles are expected to emerge in the near future. Non-aqueous electrolyte secondary batteries having a high energy density are attracting attention as a possible power source for electric vehicles, and the development thereof is proceeding.
Currently available non-aqueous electrolyte secondary batteries contain a non-aqueous electrolyte comprising a flammable non-aqueous solvent, a metal oxide (e.g., lithium cobalt oxide) as a positive electrode active material, and a carbon material (e.g., graphite) as a negative electrode active material. Accordingly, if a non-aqueous electrolyte secondary battery is heated by some factor and reaches an overheated state, the positive electrode active material decomposes and produces oxygen. If the produced oxygen oxidizes the negative electrode active material or the non-aqueous solvent, the battery might rupture or go into thermal runaway. The overheated state of non-aqueous electrolyte secondary batteries can be classified into two types: (i) an entirely heated state in which the entire battery is heated by overcharge; and (ii) a locally heated state in which the battery is locally heated by internal short-circuit. The risk of danger increases if these two states occur concurrently. In order to improve battery safety, it is necessary to prevent overcharge, as well as internal short-circuit at the area where the positive electrode active material, the negative electrode active material and the non-aqueous electrolyte exist together.
Conventionally, in order to prevent overcharge of secondary batteries, a technique is used in which a conductive material that exerts its conductivity by doping of ions is disposed between a positive electrode and a negative electrode, whereby when the battery reaches an overcharged state, the positive electrode and the negative electrode are shorted to prevent further overcharging (see, e.g., Japanese Laid-Open Patent Publications Nos. Hei 2-199769 and Hei 10-321258, hereinafter referred to as Patent Documents 1 and 2, respectively). Another conventional technique is to add an austenitic stainless steel powder to a positive electrode to allow positive and negative electrodes to be shorted, thereby preventing further overcharging (see, e.g., Japanese Patent Publication No. 3353455, hereinafter referred to as Patent Document 3).
Patent Document 1 discloses to dispose, between and in contact with positive and negative electrodes, a separator containing a polymer that exerts conductivity by doping of ions so as to cause an internal short-circuit between the positive and negative electrodes in the event of an overcharge, thereby preventing further overcharging. Patent Document 2 discloses to add, to an electrolyte, a monomer for producing a conductive polymer by polymerization at an overcharge voltage so as to allow the produced conductive polymer to cause an internal short-circuit between positive and negative electrodes, thereby preventing further overcharging. Patent Document 3 discloses to add, to a positive electrode, an austenitic stainless steel powder that dissolves at an overcharge voltage so as to allow the dissolved metal to deposit on a negative electrode to cause an internal short-circuit between the positive and negative electrodes, thereby preventing further overcharging.
According to the above-mentioned techniques, a short-circuit is caused at an area between the positive and negative electrodes where the positive and negative electrode active materials and the non-aqueous solvent exist together. However, a case can happen in which a battery reaches an overcharged state without reaching the decomposition temperature of a positive electrode active material. If the positive and negative electrodes are shorted at an area where the positive electrode active material exists while the battery is in the above condition, the short-circuit current produces Joule heat at the shorted area to further increase the local temperature to the level at which the positive electrode active material decomposes and produces oxygen. This raises internal pressure of the battery, which increases the risk of a battery rupture. Moreover, if the local temperature exceeds the ignition temperature of the non-aqueous solvent or the oxidation reaction temperature of the negative electrode active material, oxygen produced from the positive electrode active material burns them, which increases the risk of thermal runaway of the battery.
In view of the above, an object of the present invention is to provide a non-aqueous electrolyte secondary battery which is extremely excellent in safety by preventing overcharge as well as internal short-circuit at an area where a positive electrode active material, a negative electrode active material and a non-aqueous solvent exist together.